1. Field of the Invention
The subject invention relates to apparatus for sensing and numerically displaying temperature in degrees Celsius or degrees Fahrenheit.
2. Description of Prior Art
Persons skilled in the art to which the invention relates are at least constructively aware of the digital thermometer disclosed in U.S. Pat. No. 4,117,722, issued to P. M. Helmstetter on Oct. 3, 1978. The patent describes apparatus wherein a thermistor, associated with circuitry which compensates for its nonlinear temperature versus resistance characteristic, is used to sense temperature. In the apparatus, the thermistor is also associated with circuitry to provide a temperature related current to a capacitor. The current charges the capacitor until threshold sensitive circuitry is initiated to interrupt the flow of the current to the capacitor and to supply an opposing and constant current which rapidly lowers the voltage across the capacitor until a predetermined level is reached. When the predetermined level is reached, the opposing current is interrupted and the temperature related current is again allowed to flow to the capacitor. The rise time of the voltage across the capacitor is dependent on the magnitude of the temperature related current and, therefore, the charging and discharging repeats itself with a frequency related to the temperature at the site of the thermistor. The threshold circuitry includes a flip-flop which provides a signal at said frequency and the signal is coupled to a counter by a gate which is open for a first period of time if a measure of temperature in degrees Celsius is desired or for a second period of time if a measure of temperature in degrees Fahrenheit is desired. The first and second predetermined periods are related to the 9/5 ratio which relates Fahrenheit and Celsius degrees. The counter includes a selectable preset count which is used to compensate for the 32.degree. offset between Fahrenheit and Celsius temperature scales. A latch coupled to the counter drives a digital display of the seven-segment type and provides a visible indication of temperature.